Hexoses are monosaccharides with six carbon atoms. Hexoses can be classified by functional group, with aldohexoses having an aldehyde at position 1, and ketohexoses having that ketone at position 2. Aldohexoses (or aldoses) include allose, altrose, glucose, gulose, galactose, idose, talose, and mannose. Ketohexoses (or ketoses) include psicose (allulose), fructose, tagatose, and sorbose. Various aspects of these aldohexoses and ketohexoses are mentioned in the following paragraphs.
For example, D-allose (allose hereafter) is a low-calorie, natural sweetener that has ˜80% the sweetness of sucrose and is described as a noncaloric sweetening and bulking agent. It is a naturally occurring monosaccharide hexose that is present in only small amounts in specific shrubs and algae. Allose boasts several potential medical and agriculture benefits including cryoprotective, anti-oxidative, anti-hypertensive, immunosuppressive, anti-inflammatory, anti-tumor, and anti-cancer activities. It also has similar functionality in foods and beverages to sucrose. As such, allose clearly has a variety of applications in the food and beverage industries. However, due to allose's high selling prices, its use as a sweetener has been limited.
Currently allose is produced predominantly through the enzymatic isomerization of D-psicose (WO 2014069537). Overall, the method suffers because of higher feedstock cost, the costly separation of allose from D-psicose, and relatively low product yields (˜23%).
Altrose is another unnatural aldohexose and C-3 epimer of mannose. D-Altrose ((2S,3R,4R,5R)-2,3,4,5,6-Pentahydroxyhexanal) can be used as a substrate to identify, differentiate and characterize aldose isomerases such as L-fucose isomerase from Caldicellulosiruptor saccharolyticus and d-Arabinose isomerase (d-AI) from Bacillus pallidus (B. pallidus) and Klebsiella pneumoniae. Recently, sugar chains such as oligosaccharides and polysaccharides, which perform functions useful as a physiologically active substance, have attracted attention in the field of fine chemicals such as medicines and agricultural chemicals. Presently, the objects of researches on the sugar chain are restricted to those consisting of monosaccharides present in nature in large amounts and readily available to researchers, such as D-glucose, D-mannose and D-galactose. However, it is expected that various monosaccharides other than those present in nature will be required in the future in research on the synthesis of sugar chains performing more useful functions. Under the circumstances, it is highly significant and necessary to develop a method which permits preparing D-altrose, which is a rare sugar difficult to obtain, in high yield while diminishing the number of treating steps. U.S. Pat. No. 5,410,038.
D-Gulose is useful, for example, as an excipient, a chelating agent, a pharmaceutical intermediate, a cleaning agent for glass and metals, a food additive, and as an additive for detergents. U.S. Pat. No. 5,215,591.
D-galactose (galactose hereafter) is a natural sweetener that has ˜33% the sweetness of sucrose and is described as a nutritive sweetener. It is a naturally occurring monosaccharide hexose that is present in dairy products, legumes, grains, nuts, tubers and vegetables. Galactose is used by the baking industry to limit tartness and acidity in foods. Also, it is used as an energy source to increase endurance in the exercise supplement industry. In the pharmaceutical industry it is an intermediate for several medicines and is also used as a cell metabolism modulator in the optimization of protein therapeutics bioproduction. Additionally, galactose has been shown to be effective as a control agent against plant disease caused by certain plant pathogens, such as those affecting cucumber, carrot, potato and tomato plants. Due to dietary concerns (e.g. veganism) and health concerns (e.g. BSE disease) non-animal sources of galactose are of interest to industry. As such, galactose clearly has a variety of applications in the food, beverage, exercise, agriculture, and pharmaceutical industries. However, due to galactose's high selling prices, its use has been limited.
Galactose is produced predominantly through the hydrolysis of lactose (WO 2005039299A3). This method is less desirable due to a more costly feed stock and the expensive separation of glucose from galactose. Alternatively, galactose can be produced via the hydrolysis of plant-based biomass (WO 2005001145A1). This method suffers from the costly separation of galactose from the multiple other sugars released during biomass hydrolysis (e.g. xylose, arabinose, mannose, glucose, and rhamnose) and low yields (˜4.6% of the dry mass of common biomass sources is galactose).
Idose is not found in nature, but its uronic acid, iduronic acid, is important. It is a component of dermatan sulfate and heparan sulfate, which are glycosaminoglycans. (en.wikipedia.org/wiki/Idose—accessed 3/7/18).
Talose is an unnatural aldohexose that is soluble in water and slightly soluble in methanol. It is a C-2 epimer of galactose and C-4 epimer of mannose. Talose can be used as a substrate to identify, differentiate, and characterize ribose-5-phosphate isomerase(s) of Clostridia.
D-mannose (mannose hereafter) is a mildly sweet, naturally-occurring monosaccharide that is found in many fruits, vegetables, plant materials, and even the human body. Mannose boasts multiple health benefits and pharmaceutical applications. For example, mannose can be used to treat carbohydrate-deficient glycoprotein syndrome type 1b and, more commonly, urinary tract infections. Furthermore, mannose is a verified prebiotic, does not raise blood glucose levels, and shows anti-inflammatory properties. Additionally, it has been shown to enhance carcass yields in pigs and is a widely used auxiliary moisturizing agent for skin-care products. As such, mannose has a variety of applications in the pharmaceutical, cosmetic, beverage, food product, dairy, confectionery, and livestock industries. However, due to mannose's high selling prices, its use in everyday products has been limited.
Mannose is primarily produced through extraction from plants. Common methods include acid hydrolysis, thermal hydrolysis, enzymatic hydrolysis, microbial fermentation hydrolysis, and mixtures thereof. Less common methods include chemical and biological transformations. Overall, these methods are problematice due to harsh conditions, high capital expenditures, higher feedstock cost, costly separation of mannose from isomerization reactions, and relatively low product yields (15-35%).
D-allulose (also known as D-psicose) (psicose hereafter) is a low-calorie, natural sweetener that has 70% the sweetness of sucrose, but only 10% of the calories. It is a naturally occurring monosaccharide hexose that is present in only small amounts in wheat and other plants. Psicose was approved as a food additive by the Food and Drug Administration (FDA) in 2012, which designated it as generally recognized as safe (GRAS). However, due to psicose's high selling prices, its use as a sweetener has been limited. Psicose boasts a myriad of health benefits: it is low-calorie (10% of sucrose); it has a very low glycemic index of 1; it is fully absorbed in the small intestine but not metabolized and instead secreted in urine and feces; it helps regulate blood sugar by inhibiting alpha-amylase, sucrase and maltase; and it has similar functionality in foods and beverages as sucrose. As such, psicose clearly has a variety of applications in the food and beverage industries.
Currently psicose is produced predominantly through the enzymatic isomerization of fructose (WO 2014049373). Overall, the method exhibits higher feedstock cost, the costly separation of psicose from fructose, and relatively low product yields.
Fructose is a simple ketonic monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide, sucrose. Commercially, fructose is derived from sugar cane, sugar beets, and maize. The primary reason that fructose is used commercially in foods and beverages, besides its low cost, is its high relative sweetness. It is the sweetest of all naturally occurring carbohydrates. Fructose is also found in the manufactured sweetener, high-fructose corn syrup (HFCS), which is produced by treating corn syrup with enzymes, converting glucose into fructose. (en.wikipedia.org/wiki/Fructose#Physical_and_functional_propertiesU—accessed 3/7/18).
D-tagatose (tagatose hereafter) is a low-calorie, natural sweetener that has 92% the sweetness of sucrose, but only 38% of the calories. It is a naturally occurring monosaccharide hexose that is present in only small amounts in fruits, cacao, and dairy products. Tagatose was approved as a food additive by the Food and Drug Administration (FDA) in 2003, which designated it as generally recognized as safe (GRAS). However, due to tagatose's high selling prices, its use as a sweetener has been limited. Tagatose boasts a myriad of health benefits: it is non-cariogenic; it is low-calorie; it has a very low glycemic index of 3; it attenuates the glycemic index of glucose by 20%; it can lower average blood glucose levels; it helps prevent cardiovascular disease, strokes, and other vascular diseases by raising high-density lipoprotein (HDL) cholesterol; and it is a verified prebiotic and antioxidant. Lu et al., Tagatose, a New Antidiabetic and Obesity Control Drug, Diabetes Obes. Metab. 10(2): 109-34 (2008). As such, tagatose clearly has a variety of applications in the pharmaceutical, biotechnological, academic, food, beverage, dietary supplement, and grocer industries.
Tagatose is produced predominantly through the hydrolysis of lactose by lactase or acid hydrolysis to form D-glucose and D-galactose (WO 2011150556, CN 103025894, U.S. Pat. Nos. 5,002,612, 6,057,135, and 8,802,843). The D-galactose is then isomerized to D-tagatose either chemically by calcium hydroxide under alkaline conditions or enzymatically by L-arabinose isomerase under pH neutral conditions. The final product is isolated by a combination of filtration and ion exchange chromatography. This process is performed in several tanks or bioreactors. Overall, the method is disadvantageous because of the costly separation of other sugars (e.g., D-glucose, D-galactose, and unhydrolyzed lactose) and low product yields. Several methods via microbial cell fermentation are being developed, but none have been proven to be a practical alternative due to their dependence on costly feedstock (e.g., galactitol and D-psicose), low product yields, and costly separation.
Sorbose ((3R,4S,5R)-1,3,4,5,6-pentahydroxyhexan-2-one) is a ketohexose that has a sweetness equivalent to sucrose (table sugar), and it is a plant metabolite that has been found to naturally occur in grapes in small quantities. D-sorbose has been determined to be effective as a control agent of plant diseases caused by: Pseudomonas syringae pv. lachrymans and Ralstonia solanacearum. United States Patent Application Publication No. 2016/0037768.
There is a need to develop cost-effective synthetic pathways for high-yield production of the hexoses such as the aldohexoses and aldoketoses discussed above where at least one step of the processes involves an energetically favorable chemical reaction. Furthermore, there is a need for production processes where the process steps can be conducted in one tank or bioreactor and/or where costly separation steps are avoided or eliminated. There is also a need for processes of hexose production that can be conducted at a relatively low concentration of phosphate, where phosphate can be recycled, and/or the process does not require using adenosine triphosphate (ATP) as an added source of phosphate. There is also a need for hexose production pathways that do not require the use of the costly nicotinamide adenosine dinucleotide (NAD(P)(H)) coenzyme in any of the reaction steps.